1. Field of the Invention
The present invention relates to telephone system architecture for enabling a telephone answering machine (TAM) operable in a modem environment to recognize when a call originator has hung up or has otherwise been disconnected. More particularly, embodiments of the present invention enable a local modem operating in a TAM mode to return to an on-hook state and terminate the recording of any message in response to a determination that the remote caller is also back on-hook.
2. Description of Related Art
In computer-based TAM systems, incoming and outgoing messages can be recorded through a modem without a separate, external answering device. In typical arrangements, the modem is connected indirectly to the central office line via the computer, as well as to an external power source. The central office functions as a switching unit installed in the public switched telephone network (PSTN), and includes necessary operating equipment and devices for terminating and interconnecting multiple telephone lines and trunks leading to subscribers' telephone sets. The central office generally provides a nominal 48 volt power supply to the telephone in an off-hook condition across the PSTN. That is, for example, the two wires of the telephone line are connected to a battery in the central office. When the telephone handset is picked up, or goes off-hook, a standard hook switch simply connects the two wires so that current flows from the central office over one wire to the telephone and returns down the other wire to the central office.
Typically, the modem is coupled to the central office via a variety of electronic components, switches, and relays. An off-hook relay coupled between the central office and the modem connects the modem to the central office line by closing, for example, when a telephone set, coupled to the modem arrangement, is picked up, or goes off-hook. If a telephone set is not included in the modem arrangement, when the internal modem connection receives indication of an incoming call, a microprocessor controls the off-hook relay, causing it to close to connect the modem to the central office. In effect, this is analogous to picking up the telephone handset. When the off-hook relay closes, the central office is connected to the modem via the 48-volt supply which provides a DC loop current to the modem through the modem's internal data access arrangement. The DC loop current indicates that the central office line has been connected, and may also be used to power up other electronics associated with the telephone line, such as an external telephone in addition to the modem. A detailed discussion of a telephone and modem arrangement is described in copending application entitled "Data Access Arrangement with Telephone Interface," Ser. No. 08/536,886, filed Sep. 29, 1995, and invented by R. Rahamim and R. W. Frankland. The disclosure of the aforementioned copending application is incorporated herein by reference.
When a telephone call is connected by the central office, the continuing presence of both the originator (remote line) and the answering party (local line) is indicated by the maintenance of the DC loop current in the circuits on both sides. If one party hangs up, the central office, which detects the loss of the current in that party's circuit continues to monitor the current in the other circuit to detect whether the other party has also hung up. If, after several seconds have elapsed, the central office detects that the loop current continues to flow, the central office will signal the other party that the call has ended by reversing the direction of the loop current or interrupting it momentarily.
For example, when one party picks up a telephone handset, the off-hook relay closes, allowing the DC current to flow from the central office. When that first party dials a telephone number for a second party, the central office connects the first party to the second party's line, and sends back a small AC current to indicate a live ringing tone. When the remote side, or second party, picks up the telephone handset, the central office detects the flow of the remote DC loop current and determines whether the local DC loop current is still flowing. If the loop current at both sides continues to flow, the central office will connect the first and second parties together.
If either party is a TAM, when the other side hangs up, the TAM must detect that the call originator has hung up to terminate further recording. Thus, if the central office detects a lack of loop current on the call originator's side and an active loop current on the TAM side, the central office will typically interrupt the local loop current for 1.2 seconds. In some areas, the central office will interrupt the connection by reversing the voltage polarity. Traditionally, if the TAM is considered to be off-hook and thus connected to the central office during this interruption of the loop current, a recorded statement requesting the user to "please hang up" is announced.
However, since the off-hook line is just a TAM, which may have just recorded a message from a remote party, the TAM may not be able to detect the interruption of the loop current. In many TAMs, the TAM controller is programmed or otherwise caused to disconnect its own connection with the central office in response to predetermined signal levels. Typically, the determination of whether the TAM can "hear" anything from the calling line is determined by detecting the average voltage level on the line over a period of time, between particular frequencies. For example, the TAM will listen to any noises or sounds from the calling line and, according to typical voice frequencies averaged together, will look at the energy of the sounds to determine if there is no longer a voice on the calling side. However, it has been found that this type of waiting and listening also causes problems in that the period of noise or silence will be recorded before the TAM eventually stops recording and is disconnected.
Presently, for a modem operating in a TAM mode, to detect when a call originator hangs up, the modem is configured to detect "near silence" on the line for a certain time period which indicates that the message has ended, and that the remote caller has hung up. However, problems can arise when the calling party merely pauses in the middle of the message, causing the modem to hang up. Or, if the line noise is greater than the "near silence" threshold, the modem may continue to record the noise indefinitely.